Protein surface hydration mappedby site-specific mutationsWeihong Qiu*, Ya-Ting Kao*, Luyuan Zhang*, Yi Yang*, Lijuan Wang*, Wesley E. Stites†, Dongping Zhong*‡,and Ahmed H. Zewail‡§

*Departments of Physics, Chemistry, and Biochemistry, Programs of Biophysics, Chemical Physics, and Biochemistry, Ohio State University,Columbus, OH 43210; †Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701; and §Laboratory for MolecularSciences, Physical Biology Center for Ultrafast Science and Technology, California Institute of Technology, Pasadena, CA 91125

Contributed by Ahmed H. Zewail, July 25, 2006

Water motion at protein surfaces is fundamental to protein struc-ture, stability, dynamics, and function. By using intrinsic trypto-phans as local optical probes, and with femtosecond resolution, itis possible to probe surface-water motions in the hydration layer.Here, we report our studies of local hydration dynamics at thesurface of the enzyme Staphylococcus nuclease using site-specificmutations. From these studies of the WT and four related mutants,which change local charge distribution and structure, we are ableto ascertain the contribution to solvation by protein side chains asrelatively insignificant. We determined the time scales of hydrationto be 3–5 ps and 100–150 ps. The former is the result of locallibrational�rotational motions of water near the surface; the latteris a direct measure of surface hydration assisted by fluctuations ofthe protein. Experimentally, these hydration dynamics of the WTand the four mutants are also consistent with results of the totaldynamic Stokes shifts and fluorescence emission maxima and arecorrelated with their local charge distribution and structure. Wediscuss the role of protein fluctuation on the time scale of labilehydration and suggest reexamination of recent molecular dynam-ics simulations.

protein hydration � femtosecond dynamics � protein fluctuation �selective mutation

From the laboratories of the senior authors of this study(A.H.Z. and D.Z.) (1–11), there has been a series of reports

regarding the time and length scales of the water layer aroundprotein surfaces. These studies were for proteins subtilisinCarlsberg (2), monellin (3), phospholipase A2 (5), melittin (9),and human serum albumin (8, 10). A theoretical model wasdeveloped to take into account the exchange with bulk water (4,12), and the dynamics are consistent with molecular dynamics(MD) simulations of residence times (13–16) on time scales fromfemtoseconds to picoseconds. Earlier NMR studies have re-ported hydration dynamics (residence times) in the subnanosec-ond regime (17–20), but, more recently, a claim has been madethat water motions at protein surfaces are ultrafast comparedwith bulk water, only slowing down by a factor of two to three(21, 22). This �10-ps range would imply that the observedlong-time hydration dynamics in tens of picoseconds are due toprotein side-chain relaxation (22, 23). In our earlier studies (6),we addressed in detail this issue and the reasons for dominanceof hydration dynamics. To quantify the contribution of side-chain motions to total solvation on the time scale of hydration,we must carefully alter the local structure while maintaining thesame tryptophan site.

In this contribution, we report the effect of mutation (fourmutants on three site selections and the WT) on hydration of theenzyme Staphylococcus nuclease (SNase). Fig. 1 shows the x-raystructure of the protein, consisting of three �-helices and afive-stranded �-barrel with a total of 149 amino acids (24). Theonly single tryptophan residue (W140) has one edge exposed tothe surface and inserts inside the protein to form a hydrophobiccluster at the C terminus, which was found to be essential to

protein foldability, stability, and activity (25). Three surfacecharged residues, K110, K133, and E129 (Fig. 1), surround W140within 7 Å. Using an alanine scan, we replaced each chargedresidue with hydrophobic alanine one at a time by site-directedmutagenesis. We also mutated K110 into the polar residuecysteine. We measured the Stokes shifts, solvation dynamics, andlocal rigidity of the WT and the mutants, using the intrinsic W140as the molecular optical probe and with femtosecond temporalresolution. From these results, we determined the hydrationdynamics (from their correlation functions) and their depen-dence on charge distribution.

Results and DiscussionFluorescence Stokes Shifts and Femtosecond-Resolved Transients.The steady-state fluorescence emission of the WT and mutantsis shown in Fig. 2, together with the transients gated at thered-side emission (360 nm). All emission peaks are at �332.5 �0.5 nm, and no significant emission shift by the mutation ofcharged residues was observed. These striking results reveal thatthe three neighboring charged residues make a negligible con-tribution to solvation of the excited W140. These observationsare consistent with the general trend that the maximum oftryptophan emission mainly depends on the location (i.e., theextent of its exposure to surface water) and not on its neigh-boring residues (26). Thus, the observed Stokes shift cannot bedue to neighboring residues’ (with charges) solvation, indicatingrelatively immobile charged side chains. The Stokes shift isdominantly due to hydration. From the x-ray structure (Fig. 1),we find that W140 is sandwiched between K110 and K133 (4.65and 2.99 Å, respectively), forming two cation–� interactions.K133 is only 3.50 Å from E129, resulting in formation of a saltbridge. This unique structural motif with these strong electro-static interactions is probably the origin of local protein rigidityaround W140, which is consistent with our observation, asdiscussed below.

Fig. 3 shows the femtosecond-resolved fluorescence transientsof W140 in the WT for several typical wavelengths, from the blueto red side, and for �10 gated emissions. The overall decaydynamics is significantly slower than that of aqueous tryptophanin a similar buffer solution (7). Clearly, the ultrafast decaycomponents (�1 ps) observed in tryptophan solution were notobserved at the blue side for the protein. Besides the lifetimecontributions, solvation components for all blue-side transientsare well represented by a double-exponential decay with timeconstants ranging from 4.9 to 12 ps and from 102 to 130 ps. For

Author contributions: D.Z. and A.H.Z. designed research; W.Q., Y.-T.K., L.Z., Y.Y., and L.W.performed research; W.E.S. contributed proteins; W.Q., Y.-T.K., and L.Z. analyzed data; andD.Z. and A.H.Z. wrote the paper.

The authors declare no conflict of interest.

Abbreviations: SNase, Staphylococcus nuclease; MD, molecular dynamics.

‡To whom correspondence may be addressed. E-mail: dongping@mps.ohio-state.edu orzewail@caltech.edu.

© 2006 by The National Academy of Sciences of the USA

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the red-side emission, the rise occurs in the range of 1.0–4.8 psand is clearly present in all transients. For the four mutants,K133A, K110C, K110A, and E129A, besides the different life-time emission contributions, the transients showed similar sol-vation patterns from the blue side to the red side (see Fig. 6,which is published as supporting information on the PNAS website). Among the four mutants, K110A has the shortest decaytime for the two solvation components, followed by E129A. Themutants K133A and K110C have similar temporal behaviors asthe WT.

Solvation Correlation Functions. Using the methodology we recentlydeveloped (7), we constructed the overall and lifetime-associated,femtosecond-resolved emission spectra (FRES) for the WT and allfour mutants. By fitting these FRES to a lognormal function, wededuced the femtosecond-resolved overall emission maxima �s andlifetime-associated emission maxima �l (see Fig. 7, which is pub-lished as supporting information on the PNAS web site). Theobtained total dynamic Stokes shifts are 850 � 50 cm�1 for all of

the proteins, and the emission maxima at t � 0, �s (0) are all near321 nm, consistent with a previous report (27) giving the emissionpeak of several proteins as 320 nm at 2 K and 300-nm excitation.MD simulations of the Stokes shift of tryptophans in four proteinsalso gave an emission maximum of �320 nm at t � 0 (28). Thepossible contribution of vibrational relaxation (9–11) is negligible,and thus the observed total Stokes shift is dominantly from the localsolvation.

Using c(t) � [�s(t) � �l(t)]�[�s(0) � �l(0)] (7), we constructedall solvation correlation functions, and the obtained results areshown in Fig. 4 and summarized in Table 1. All solvationcorrelation functions can be represented by a double-exponential decay. For the WT, the time scales are 5.1 ps with46% of the total amplitude and 153 ps (with 54% of the totalamplitude); for K110C, 4.2 ps (51%) and 149 ps (49%); forK133A, 3.9 ps (59%) and 157 ps (41%); for E129A, 3.5 ps (60%)and 124 ps (40%); for K110A, 3.1 ps (77%) and 96 ps (23%).Overall, all four mutants show faster temporal behaviors thanthe WT, and all of the second long solvation times are within

Fig. 1. Protein structure with sites of hydration. (Left) X-ray crystallographic structure of WT SNase (Protein Data Bank ID code 1SNO). The single tryptophanW140 (in yellow) is sandwiched between K110 and K133, with one edge exposed to the protein surface. (Right) The local configuration around W140 with threecharged residues (K110, K133, and E129) in close proximity (�5 Å).

Fig. 2. Emission spectra and transients of the proteins. (Left) Normalized steady-state fluorescence spectra of WT SNase and four mutants, E129A, K110A, K110C,and K133A. The mutant K110A has the strongest fluorescence intensity, and K110C has the weakest one because of the quenching by C110. Note that nosignificant emission shifts were induced by the mutation of charged residues. (Right) Normalized femtosecond-resolved fluorescence transients of W140 fromWT SNase and all four mutants at the red-side emission (360 nm).

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�100–150 ps. Note that no ultrafast solvation dynamics in �1 pswere observed for the proteins studied here.

Hydration vs. Charged Side-Chain Solvation. The constructed sol-vation correlation function is the response of the local environ-ment around W140 to the sudden dipole change from the groundstate to the excited state. In principle, the response results fromboth surrounding water molecules and neighboring proteinpolar�charged residues (and protein peptide bonds). If thesecond long-time solvation came only from protein side-chainmotions (22, 23) (in this case, from three neighboring chargedresidues), we should observe significantly different solvationtime scales and amplitudes of the second solvation components.However, for the mutants K133A and K110C, we observed a longrelaxation time (149 and 157 ps) similar to the WT (153 ps).Overall, the obtained solvation dynamics for the four mutantsbecome faster with the decrease in the local charge distributionaround W140 (Fig. 4), consistent with MD simulations that showthat water has longer residence times around charged residuesthan near hydrophobic side chains (15, 29). The observed longtime scale of �100–150 ps also is consistent with our recentobservation in the melittin tetramer and human serum albumin(E and F isomers); in both cases (9, 10), the probe tryptophanhas a similar emission maxima (�332 nm) and charge surround-ings as in SNase. Thus, the obtained solvation correlationfunctions essentially reflect the local hydration dynamics at theprotein surfaces.

For SNase, W140 has one edge exposed to the surface.Because of the local strong electrostatic interactions of thecation–� stacking�salt bridge and the tight packing of thehydrophobic cluster near the C terminus, the local structure ismore rigid and W140 is less mobile. We studied the femtosec-

ond-resolved rotational dynamics of W140 by the measurementof anisotropy changes with time for all of the proteins (see Fig.8, which is published as supporting information on the PNASweb site). We observed that all anisotropy dynamics dominantlydecay in nanoseconds with a small decay component at the earlytime. The long nanosecond dynamics represents the whole-protein tumbling motion. The early small decays, on the timescales from 62 to 464 ps, result from the local wobbling motionswith semiangles of 11–19o (30). The time scales of 62–464 ps donot correlate with those of solvation dynamics (Fig. 4). All ofthese findings suggest a relatively small local f luctuation; theprotein structure around W140 does not undergo large confor-mation changes in the time window of our measurements (�1.3ns). These results from anisotropy dynamics are consistent withthose obtained above from site-selected mutagenesis. The con-clusion is indeed consistent with the results of the steady-statefluorescence emission maxima (i.e., the dynamic Stokes shifts ofthe WT and four mutant proteins are independent of the threecharged residues K110, K133, and E129). However, these find-ings are contrary to those of recent MD simulations, whichindicate that, for this protein (SNase), K110 and K133 effectscombine to make a very large contribution to the total Stokesshift (28).

Molecular Mechanism of Surface-Water Hydration. To summarize,the studies of mutations, steady-state fluorescence, anisotropymeasurements, and biphasic behavior of dynamic Stokes shiftsexclude charged side-chain solvation as a significant contributor.As discussed in ref. 6, in all previous work reported from our

Fig. 3. Normalized femtosecond-resolved fluorescence transients of W140from WT SNase on short (Left) and long (Right) time scales with a series ofgated fluorescence emissions. Note that no ultrafast decay components (�1ps) were observed.

Fig. 4. Hydration correlation functions probed by tryptophan W140 for WTSNase and four mutants, K110C, K133A, E129A, and K110A. Inset shows thecorrelation functions in the short time range. Note the similarity of the timescales and the difference in amplitudes.

Table 1. Results obtained from the hydration correlationfunctions c(t) of WT SNase and four mutant proteins

SNase �1 �2 c1 c2

WT 5.1 153 0.46 0.54K110C 4.2 149 0.51 0.49K133A 3.9 157 0.59 0.41E129A 3.5 124 0.60 0.40K110A 3.1 96 0.77 0.23

All of the hydration correlation functions were fitted with c(t) � c1e�t1��1 �c2e�t2/�2, where c1 � c2 � 1. The time constants are in picoseconds.

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laboratories, the robustness of the temporal behavior for differ-ent proteins of different sequence and structure supported theabove conclusion, which is now reexamined with site-selectedmutagenesis of the same protein. We did not observe a signif-icant ultrafast component (�1 ps) for any of the proteins studiedhere, contrary to the initial dynamics obtained from recent MDsimulations (13–15, 23). Given that we were able to resolve the200-fs solvation dynamics of tryptophan in bulk water (1, 7), weshould have been able to detect subpicosecond dynamics presentin the proteins studied. The absence of subpicosecond solvationsuggests that the force field used in the MD simulations mightbe too flexible and should be reexamined to account for relevantinteractions between the water network and protein.

Previous MD simulations gave a biphasic distribution ofresidence times with two discrete time scales (13–15), whichshow remarkable similarity with our obtained hydration dynam-ics. Two distinct residence times represent two different ways forwater to escape hydration sites. The fast route is throughconsecutive hopping by breaking and making of the neighboringhydrogen bonds, and the slow path is through direct exchangewith bulk water by disrupting the local water structure: hence,the distribution in residence times (31). Along the slow path,water has longer residence times at concave surfaces than aroundconvex patches, and around charge residues than near hydro-phobic side chains (32). Our recent MD simulations of nonequi-librium excited-state trajectories of tryptophan in proteins givea similar dynamic pattern of two distinct time scales in additionto a subpicosecond component (T. Li, A. Hassanali, D.Z., andS. Singer, unpublished data). The initial dynamics occurs in a fewpicoseconds and represents local water librational�rotationalmotions as discussed before. The long-time solvation is alsoobserved in tens of picoseconds from local water collectivemotions but is strongly assisted by local protein fluctuations.Only if the local protein structural rigidity is lost by conforma-tional changes does the protein contribute to the total solvationenergy in a significant way (33). It should be noted that staticspectral shifts reflect the internal and static Stark perturbation,which attractively or repulsively alters the energy levels atequilibrium, whereas dynamical solvation measures the fluctu-ations in energy as a function of time (6).

Fig. 5 shows a series of surface maps around W140 for the WTand four mutants with the local surface topography and neigh-boring residues. The x-ray structure at 1.7-Å resolution showsaround W140 four surface-water molecules sticking to threecharged residues and one water molecule buried inside theprotein. As also shown by recent MD simulations of waterpenetration in SNase (34), the buried water molecule bridgesthree residues, the side chain of W140 (NE1–O distance of 2.87Å) and two backbone oxygen atoms of V104 (O–O distance of2.89 Å) and A109 (O–O distance of 2.6 Å), by three hydrogenbonds and has a residence time that is longer than the simulationtime window of 10 ns. Another recent 7-ns MD simulation (35)of surface-water properties reveals a complex hydration struc-ture with a pronounced pentagon–pentagon ring distribution inthe hydration layers that depends highly on the surface topog-raphy. Around the hydrophobic atoms, clathrate-like cage struc-tures parallel to the protein surface were observed. Near thepolar atoms, the ring structures, perpendicular to the surface,have one or two hydrogen bonds with the polar atoms. From ourrecent 2-ns MD simulations, we found that within 5 Å from theO atom of H2O to the indole moiety (including H), there are �17water molecules around the ground-state W140. We did notobserve significant changes of total water molecules for thedifferent mutants, implying nearly similar polar water environ-ments, consistent with our observation of the same Stokes shiftsfor all mutants and the WT.

When tryptophan is excited to the 1La state, the local equi-librium is shifted, and the system is in a nonequilibrium state.

Our observed hydration dynamics, with two distinct time scales,reflect the temporal evolution of two types of motions from theinitial nonequilibrium configuration to new equilibrated statearound the excited tryptophan. The initial hydration dynamics,occurring in 3–5 ps, represents the librational�rotational mo-tions of these surrounding water molecules, slowing down by afactor of three to five compared with similar bulk-water motionsaround tryptophan, which occur in �1 ps (1, 7). We clearlyobserved a correlation of the initial fast time scale and amplitudewith the local charge distribution (Table 1). For example, weobserved the fastest hydration dynamics, in 3.1 ps, for the mutantK110A with the largest amplitude (77%); for K110C, we ob-tained a time scale of 4.2 ps with 51%; for the WT (K110), wehave the initial dynamics of 5.1 ps with 46%. Thus, from thehydrophobic (A110) to the polar (C110) and to the chargedresidue (K110), we observed a lengthening of the time scale,consistent with stronger interactions of local water with charges.Furthermore, we observed a decrease, not an increase, in theamplitude, suggesting that the charged�polar residue does notdirectly contribute to the solvation energy. These results revealthat the initial relaxation is from the local surface-water motionsand that the observed variations in hydration dynamics in themutations reflect the alternation of the local landscape andthe change of the neighboring chemical identity (Fig. 5).

The observed slow water dynamics in 100–150 ps represent the

Fig. 5. Protein surface maps of the WT and four mutants showing the localtopography and neighboring protein residues around W140. There are fourwater molecules sticking to three surface charged residues (K110, E129, andK133) near the probe W140 in the x-ray structure. The water molecules within5 Å from the O atom of H2O to the indole ring are shown from our 2-nssingle-trajectory MD simulations at 295 K in aqueous solution. Note that thenumber of water molecules is nearly the same for all of the proteins but thestructures are different. The structural landscapes are different, especially forE129A and K110A, with ruptures at the surfaces near W140.

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long-time collective motions, reflecting a dynamic structuralchange of the local water molecules. The time scale to reach thenew structural configuration depends on the local protein–waterinteractions. Upon the sudden change of the dipole moment ofW140 in the protein, besides initial librational�rotational mo-tions, at least two local structural motions are expected: one isthe alignment of the local water network�cluster to the newexcited-state (1La) dipole moment, and the other is the increaseof water molecules around W140 due to the larger excited-statedipole moment (�� � 5 debye units). Because of the longer timescale (tens of picoseconds), these overall structural changes arecoupled with the protein fluctuation. These fluctuations assist instructural rearrangement of surface water and its exchange withbulk water. As such, the connection of residence time to hydra-tion is incomplete without knowledge of fluctuations. Thus, thelong-time hydration dynamics, defined here as the rate for waterstructure to reach the new equilibrated state of minimal freeenergy, is an integrated process determined by the local inter-actions with the protein (Fig. 5) and assisted by its f luctuations.The protein–water coupling motions enable surface-water mol-ecules to make structural arrangements (solvation), but thesesmall protein fluctuations themselves do not make direct con-tributions to total solvation.

ConclusionThe studies reported here, using site-selected mutagenesis of theenzyme SNase, indicate that surface protein hydration occurs onthe picosecond time scale with a biphasic behavior, consistentwith observations made previously for a number of otherproteins that we have studied. The fastest component (a fewpicoseconds) is due to the librational�rotational motions ofwater near the surface, whereas the longest one (tens of pico-seconds) is due to the water molecules coupled to the surface andstructurally modified by protein fluctuations. The evidence forhydration, in contrast to the dominance of side-chain solvation,comes from a number of observations discussed above and in ref.6 in studies of different proteins, but here the evidence comesfrom studies of the same protein but with different side chainsof different charges and structures. It would be fortuitous, andit seems unlikely, that mutations at three different sites wouldexactly cancel out the effect of hydration and protein solvationto give nearly the same Stokes shifts and correlation functions.For another protein, the insignificant contribution of side-chainsolvation was recently examined in experiments involving thenative and denatured states (36).

The key to understanding surface hydration is the time scalesinvolved. In a simple model (for reviews, see refs. 4, 6, 12, and37), the time scale is related to hydrogen-bond exchanges inlayered water, consistent with residence times obtained fromseveral MD simulations (6, 13). Such a picture can explain slowhydration also in micelles (37). However, on time scales of a fewpicoseconds, proteins are ‘‘rigid,’’ whereas on time scales of tensof picoseconds, they fluctuate. This separation of the time scalesis relevant not only for hydration but also for other proteininteractions, such as molecular recognition (38). The fluctua-tions assist solvation by allowing for restructuring in the newnonequilibrium state (in this case, the structure of water aroundtryptophan’s dipole). They themselves do not contribute directlyto solvation by minimizing energy, as evidenced by the robust-ness of hydration time scales and Stokes shifts independent oflocal charge and structural changes around tryptophan. Large-amplitude side-chain motions may occur, but they are on some-what longer time scales and may involve conformation changes;such changes contribute to solvation (33).

Because it directly represents interactions with the protein,unlike libration or distant rotation, the longer picosecond hy-dration discussed here is significant for biological function (forrecent reviews, see refs. 39 and 40). The picosecond time scale,

as pointed out in ref. 6, is ideal for many biological processesinvolved in molecular recognition, reactivity, and conforma-tional intactness. The robustness in longer-time hydration forproteins of different sequence and structure cannot be ignored.In earlier NMR work, it was shown in a series of papers (forreviews, see refs. 17–20) that the hydration of proteins and DNAoccurs in subnanoseconds, and it was the work from our labo-ratories (for review, see refs. 6 and 9) that pointed out that thetime scale of surface hydration is much shorter than the timescales reported by NMR: namely, from 1 ps to tens of picosec-onds (biphasic) with femtosecond resolution. However, in morerecent work published from one of the same NMR groups(21–23), it was ascertained that hydration occurs in a fewpicoseconds. Similarly, in earlier MD simulations (see, forexample, ref. 13), the time scales varied from picosecond tonanosecond residence times, depending on the strength ofhydrogen bonding and the location of water molecules, but witha force field and linear response theory, the claim now is thatwater is much more labile, although fast and slow (10%)components were present, with the latter assigned to proteinsolvation (23). As discussed above, the search for the largeultrafast (�1 ps) dynamics predicted by MD simulations wasunsuccessful. It is possible that the flexibility imposed by MDmodeling is reducing all time scales and increasing amplitudes ofsubpicosecond components by order(s) of magnitude. Proteinfluctuations, including side chains, cannot be blind to local waterhydration that is responsible for solvation and is structurallyevident through x-ray and mutation studies. It is concluded thatthe picture based on MD simulations and emphasized in severalrecent publications (22, 23) needs to be revisited, taking intoconsideration the approximations made regarding force field,linear response, and the exact meaning of hydration for thecoupled protein–water network. Protein fluctuation-assisted in-teractions with surface water are an integral part of hydrationdynamics, particularly at longer times.

MethodsFemtosecond Methods. All experimental measurements were car-ried out by using the femtosecond-resolved fluorescence up-conversion apparatus described in refs. 11 and 41. Briefly, thepump laser was set at 290 nm, and the energy was typicallyattenuated to �140 nJ before being focused into the motor-controlled moving sample cell. The fluorescence emission wascollected by a pair of parabolic mirrors and mixed with a gatingpulse (800 nm) in a 0.2-mm �-barium borate (BBO) crystalthrough a noncollinear configuration. The up-converted signalranging from 218 to 292 nm was detected. The instrumentresponse time under the current noncollinear geometry is be-tween 400 and 500 fs as determined from the up-conversionsignal of Raman scattering by water at �320 nm. Most mea-surements were carried out at the magic-angle (54.7o) condition.For fluorescence anisotropy measurements, the pump-beampolarization was rotated to become either parallel or perpen-dicular to the BBO acceptance axis to obtain the parallel (I�) andperpendicular (I�) signals, respectively. These transients wereused to construct the time-resolved anisotropy: r(t) � (I� �I�)�(I� � 2I�).

Protein Site-Selected Mutagenesis and Preparation. The four mu-tants of SNase, E129A, K133A, K110A, and K110C, wereprepared by the method of Kunkel as described in ref. 42. Proteinexpression and purification were performed by following theprocedure described in ref. 43. The obtained proteins werefinally dissolved in a buffer of 25 mM sodium phosphate and 100mM NaCl at pH 7. The protein concentration used in femto-second-resolved studies was 200–300 �M. The fluorescenceemission was measured by using a SPEX FluoroMax-3 spec-trometer. To ensure that no change of the protein quality

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occurred during data acquisition, we measured the proteinfluorescence emission before and after experiments. We alsokept the sample in a rotating cell to minimize possible photo-bleaching.

Mutant Lifetime. The lifetimes of all five proteins were deter-mined by gating the relaxed-state emission at the red side of the360-nm emission. The obtained transients are shown in Fig. 2Right. For the WT, the transient was best fitted by a double-exponential decay with time constants of 1.2 and 5.6 ns, consis-tent with previous frequency-domain measurements (44). Be-cause the time interval between two consecutive pumpexcitations was 2 ms and each transient was averaged over �1 h,all f luctuating protein configurations were probably sampled onsuch long times. Thus, the apparent two lifetimes do not meanthat only two static configurations are present in solution but thattwo types of temporal configurations are present; the system isdynamically heterogeneous on a time scale longer than a fewnanoseconds.

We obtained a similar transient for the E129A mutant,indicating negligible excited-state quenching by the chargedresidue E129. For the K110A mutant, a nearly single-exponential decay was observed with a lifetime of 8.7 ns,indicating minor dynamic quenching by immobile K133 andconsistent with a strong cation–� interaction at 2.99 Å.

Without any quenching in proteins, tryptophan emission has atypical lifetime of �9 ns (26). For the K133A mutant, thetransient becomes faster, and the lifetimes shorten to 0.9 and4.5 ns, suggesting a noticeable quenching by K110 due to a‘‘loose’’ cation–� interaction at 4.65 Å. The mutant K110C hasthe shortest lifetimes, 0.8 and 3.5 ns, and has stronger quench-ing than K133A due to the quenching by C110 through anelectron-transfer mechanism (45). These obtained lifetimescorrelate with neighboring chemical identities (46), separationdistances, and observed f luorescence intensities and were usedin the data analyses above. It should be noted that althoughchanges in intensities and lifetimes ref lect quenching, thesequenching dynamics are on the nanosecond time scale, and theblue-side emission wavelengths are more sensitive to solvation,which is typically on the picosecond time scale (11, 47).

We thank Prof. Patrik R. Callis for catalysis of the initial collaborationfor the work with W.E.S. and for constructive comments on themanuscript, Bradford Bullock for help with preparing protein samples,Prof. Sherwin Singer for helpful discussion, Tanping Li for efforts inpreparing Fig. 5, and Dr. Wenyun Lu for initial help with theexperiments. This work was supported by Petroleum Research FundGrant PRF-42734-G4, the Packard Foundation Fellowship (to D.Z.),National Science Foundation grants (to D.Z. and A.H.Z.), andNational Institutes of Health Grant NCRR COBRE P20 RR15569(to W.E.S.).

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